Many years ago I read a sci-fi book where it followed the activities of a very wealthy family that would make investments on different planets and then (due to time dilation of close to light speed travel) return to the original planets many hundreds of years later (and be many billions of creds richer).
Anyone know of what book this was?
 I read the book sometime in the 70s. It was at least a decade or two before I read Ender's Game, so that wasn't it.
I checked the Wiki for the other books mentioned below. So far none are even close - sorry (but good choices). Forever War also has compound interest from unpaid salaries but that wasn't it either.
And the financial empire's various investments was a major part of the plot.
I'll post if I find the answer.[/edit]